[demos] Improved text-on-path glyph alignment

Instead of aligning the bottom-left corner, align the bottom-mid point
with the path tangent.

Change-Id: I0274f20adafd8de4e1ccc141535005bd7e00083b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1294776
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
Auto-Submit: Florin Malita <fmalita@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>